Heater for indirectly heated cathodes



Jan. 2, 1934. A. H. YOUNG HEATER FOR INDIRECTLY HEATED CATHODES Filed Au27, 1931 Fig. l

. Inventor: Andrew H.Youn2,

by Hus Attorney.

Patented Jan. 2.1934

PATENT OFFICE mm mm mmnncrnr HEATED cs'rnonss Andrew H. YoungSchenectady, N. Y., assignor a.

to General metm of New York Company, a corporation Application All!!!27, 1931. Serial No. 559,758

3 Claims. (01. etc-27.5)

The present invention relates to thermionic apparatus and moreparticularly to the cathodes thereof.

There has arisen the necessity for a cathode,

6 structure which may be energized directly on commercial voltages suchas 110 volts, without requiring transformers, resistances and the like.Attempts in the past to design a satisfactory heater with this voltagerating for indirectly l heated cathodes have metwith partial failure,

not only from the standpoint of life and performance but also from thatof production on a quantity basis. The requirements for a member of thissort are extremely critical on account of 'the limited space availablein standard tube design which ordinarily precludes a filament of suchlength as is required at these high voltages and also on account offailing to allow sumcient space for the proper amount of insulation. The

combination of high voltage with the resulting high temperature ofoperation and the closeness of the filament turns made necessary by thesmallness of the containing member have often caused short circuitbetween the turns even through intermediate insulation, and inpractically all cases in which ordinary insulation material has beenemployed, severe electrolysis has taken place and destroyed thematerial. These failures are particularly common inthe 80 case ofgas-filled devices in which local .dis-

charges tend to form about various portions of,

15 employed heater members inseparable from the cathode and theirconstruction is such as not.

to allow ready substitution or renewal of the heater.

An object of the present invention is to provide a heater which may beenergized directly from commercial voltages and which does not have theforegoing disadvantages, but on the contrary, has a long andsatisfactory operating life, and is also adapted to quantity productionmethods. Other objects are to provide an indirectly heated cathode witha readily replaceable heater, also to provide a heater which may bemanufactured as a member, complete in itself and separate from thecathode proper. These objects are attained in brief, by coiling theheater wire in a fine helix and winding the latter about an insulatordesigned to withstand electrolysis. The heater wire is held rigidly inplace by the application of one or more coats of an insulating pastecontaining a material highly resistant to electrolysis at hightemperatures and voltages, and which when dry, forms a hard, solid mass.The heater in the finished form is an article complete in itself.self-supporting, and is embedded in an insulating material whicheffectively precludes arcing between the turns and protects the enclosedmetal member from injury. The invention will be betterunderstood whenreference is made to the following specification and accompanyingdrawing in which Fig. 1 illustrates an exemplary tube which employs theimproved heater to advantage; Fig. 2 is an enlarged view showing apreliminary step in the manufacture of the heater; Fig. 3 is a view ofthe heater infinal form and mounted in place, ready to be inserted intothe cathode casing; Fig. 4 is a view partly in section of the heatershown in Fig. 3, while Fig. 5 is a sectional view of an improved cathodetaken along the line 5-5 of Fig. 1.

In Fig. l, numeral 1 designates the envelope of an electrostaticallycontrolled arc discharge device embodying a tipless seal so-called, andcontaining an indirectly heated cathode 2, an electrostatic control orgrid member 3 of mesh material and an inverted dish-shaped anode 4. Adevice of this sort ordinarily contains an ionizabie medium such asmercury or an inert gas which in operation, reduces the space chargeeffect to such an extent as to allow large amounts of current to flow,controllable by the charge on the grid. Alternating voltage is usuallyimpressed between the cathode and anode and is rectified within thetube.

The cathode structure 2, shown more clearly in Fig. 5, consists ofquadrant pieces of sheet metal bent to a shape so that when fittedtogether, two concentric cylinders 8 are formed, separated by fourradially extending vanes 7. These vanes, as well as the two cylinders,are coated with an electronically active substance, such as bariumcarbonate (311003). There are several heat-shielding cylinders 8surrounding the outer cylinder 6 and separated therefrom by depressions9 formed in the material of the shields.

The improved heater which forms the subject of the present inventionfits snugly within the inner cylinder 6 and serves to heat the latter,also the outer cylinder and vanes, to an electronemitting temperature inthe well understood manner. The heater is constituted preferably oftungsten wire 10 of relatively small gauge, coiled as a fine helix, asshown more clearly in Fig. 2. The coil is wound in a cold state, as acoarse helix 11o aboutaroundrodorcore 11 of highlyfired alumina (AhOa)but which may also comprise zirconia (ZrOz) beryllia (BeO), or otherinsulating refractory which resists electrolysis to a high degree. Thereis a stiff molybdenum wire 12 driven through a central opening in therod to which wire the upper end of the heater is attached so as to serveas a combined lead and tie wire. The lower end of the heater coilconveniently is bound to the alumina rod by a wire thong 13 which may beextended to constitute the other lead. The combined coil and its rodsupport are first dipped in water and then in a coating materialconsisting of a suspended solution of starch paste and fairly coarse,highly fired, pure alumina powder. While I prefer alumina for thispurpose, other refractory insulating materials such as beryllia andzirconia, may be employed, provided they are inert with respect to thematerial of the embedded filament at high temperature and are highlyresistant to electrolysis, also are capable of being thoroughlydegassed; The slightest trace of gas in the material causes a severeshortening of the heater life. The coating is dried in the open air orin a closed oven operated at about .100 C. or less, and preferably isaccomplished in such a manner as to insure uniformity of drying. Thedipping process allows a very even coat to be picked up from thesolution and as the water evaporates, the particles are drawn togetherby surface tension and the adhesive effect of the starch to form arelatively hard and compact mass. When the first coating is dry, theheater is again dipped in the suspension and dried in this manner untilthe proper thickness of material has been built up. A complete coat maybe obtained with only two dips by regulating the condition of thesuspension. It is desirable to have an extra heavy coat at the top ofthe heater which ordinarily is the point of maximum voltage between theheater and the cathode, and for this reason the upper portion of thecoated member is given one or more extra dips.

The next step is to fire the heater, which is carried out at atemperature between 1575 C. and 1600 C. in an atmosphere of moisthydrogen. This firing step serves not only to get rid of the gas andvolatile portion of the binder material and to decarbonize the lessvolatile portion of the binder, but also to sinter the coating. Thefilament heater is thus encased in a thick layer of pure aluminum oxideA10: which takes on an extremely hard, sintered character. The member asa whole is rugged, self-supporting, and is capable of being handledreadily. It is also apparent that the described process of manufactureof the heater is relatively simple, and lends itself to quantityproduction methods.

The heater is now ready to be assembled within the cylinder or casing ofthe cathode, and in order to obtain a snug fit, it may be necessary tosandpaper the exterior of the coating. However, in practice, theconstituency of the dipping solution and the number of dips are soregulated as to avoid the necessity of any paring of the coatingmaterial. The heater is supported in a vertical position by means of acombined cap and collar member 14 which has an opening in the top toreceive the heater, and which fits snugly over a piece of glass tubingas a pant leg 15 sealed to the press 16 of the envelope 1. To ensure arigid seat for the heater in the cap, the lower end of the heater membermay be tapered slightly. The central wire 12 is passed through the glasstubing and is connected to one of the not only by the proper choice ofinsulating mabetween the filament and the cathode, so that external basecontact 17. from the heater is secured in anynertoarigldconductorlafused and connected to another of the 17. Thebottom of the cathode structure is closed by a metal cap (not shown) andthe memberasawholesupportedfromthepreabya rigid metal rod 19, inaddition to the conductor 18, which on account of being attached tothecap serves as a lead for the cathode as well as for the heater. As shownin Fig. 1, the cathode preferably is mounted so as to extend a shortdistance within the grid member and is spaced concentrically therefromby means of a surrounding disc of insulation material 20. In addition tothe cathode, the press also supports the grid 3 conveniently by means ofstay rods 21 secured to a clamp 22.

As stated hereinbefore, a tube of this sort contains an ionizable mediumsuch as mercury v r, or an inert gas at a pressure sufiicient to su portan arc-like discharge at the impressed voltages. It has been found thatnotwithstanding the presence of the medium in this condition, also thehigh voltages employed, any portion of the heater which is accessibleto' the arc successfully withstands the positive ion bombardment. Thismay be due to the hard and dense texture of the fused coating. Theheater may be energized directly from a 110 volt alternating current ordirect current supply without the use of voltage reducing accessories,as the size and length ofthewirearesuchastolimitthe currentat thesevoltages to the proper amount. It has been found that even when heatedto a tempera ture as high as 1300 C., the improved heater is not subjectto electrolysis which ordinarily attacks all other heaters attemperatures of this order. This desirable result is brought aboutterlals in which the wire is embedded and 011' which it is wound, butalso by the improved process of manufacture and design of the heater. Itis known that the degree with which the phenomenon of electrolysis takesplace in a given ma-- terial depends not only upon the electrical stressto which the material is subjected but also on the temperature of thematerial. The improved heater is so designed as to minimize thistemperature, the design being an embodiment in which the heater is woundas a double helix on a rod of suitable insulating material. Moreover. itis apparent that this electrical stress is relatively high due to thepresence of 110 volts between the adjacent legs of the filament and alsothe problem of eliminatingelectrolysis of which the slightest amount isfatal to the life of the heater, is a matter of no small importance whenthe thinness of the encasing material is considered. I have made110-volt heaters of this sort with a coating of approximately 25 milsthick, at the thinnest part which operated as long as 4450 hours in amercuryvapor discharge device of standard design. The maximum diameterof this 140 heater was 170 mils, approximately the same size as shown inFig. 4 of the drawing. The member 14, which is completely insulated byreason of its glass support and also by virtue of the fact that only theinsulating cover of the heater con- 5 tacts with it, serves to preventarcing between the lower end of the wire 12 and of the helix 10 betweenwhich full heater voltage is applied and also precludes positive ionbombardment of the wire which tends to take place in the gas.

While I have described my invention with particular reference to a gasor vapor-filled rectifier, it is to be understood that the featuresdescribed hereinbefore are applicable to all forms of gaseous dischargedevices, such as glow lamps and the like. Indeed, the invention is notlimited even to gas discharges, but may be extended into the high vacuumfield because obviously, an in directly heated cathode which is able tooperate satisfactorily under the severe conditions imposed by cumulativeionization and high voltage, is readily adapted to the less severeconditions occurring in a high or partial vacuum. Consequently, theinvention applies to all types of electric discharge devices, gas-filledor highly evacuated, which employ indirectly heated cathodes and whichadvantageously may use a heater connected directly to a commercialvoltage supply.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. An indirectly heated cathode for a gasfllled tube and adapted to beenergized directly by current of commercial voltages, said cathodecomprising a heater wire wound on a rod of refractory insulatingmaterial and completely embedded in a mass of aluminum oxide, a heaterlead extending through the rod, said mass or aluminum oxide resting on ametal cap which is insulatingly supported within the tube. said leadpassing through the interior of said cap.

2. An electron discharge device comprising an envelope terminating in astem, cooperating electrodes in said envelope including an indirectlyheated cathode adapted to be energized by current of commercialvoltages, said cathode comprising a heater wound on a rod 01' refractoryinsulating material and completely embedded in a mass of refractoryinsulating material, a heater lead extending through the rod, a collaror refractory material for supporting said mass of insulating materialfrom said stem, said lead passing through said collar to the exterior ofthe envelope.

3. An electron discharge device comprising an envelope terminating in astem provided with a cylindrical projection, cooperating electrodes insaid envelope including an indirectly heated cathode adapted to beenergized by current of commercial voltages, said cathode comprising aheater wound on a rod of refractory insulating material and completelyembedded in a mass of refractory insulating material, a heater leadextending through the rod, a metal collar fitting snugly over saidprojection and over said mass of insulating material for supporting thelatter from said stem, said lead passing through the collar to theexterior of the envelope.

ANDREW H. YOUNG.

